There is presently a significant amount of research being conducted in the field of thermal wave microscopy. In thermal wave microscopy, a periodic heat source is focused on the surface of a sample. The heat source is typically supplied by either an intensity modulated laser beam or a stream of particles, such as an electron beam. When the sample absorbs the incident energy at or near the sample surface, a periodic surface heating results which, in turn, generates thermal waves that propagate from the irradiated spot. These thermal waves have the same frequency as the beam modulation frequency. The wavelength of the thermal waves is determined both by the frequency of the beam and by the thermal parameters of the sample.
In a thermal wave microscope, thermal features beneath the sample surface are detected and imaged by sensing the thermal waves that scatter and reflect from these features. The thermal waves are highly damped such that they travel only one or two wavelengths before becoming too weak to detect. Nevertheless, a variety of methods have been developed capable of sensing and measuring the thermal waves generated in the sample.
One method of detection includes the sensing of acoustic waves which are generated by the thermal waves. More particularly, acoustic waves are generated because the thermal waves induce stress-strain oscillations in the heated region of the sample. These elastic waves are true propagating waves and can be detected with conventional ultrasonic transducers. This technique disclosed in U.S. Pat. No. 4,255,971, issued Mar. 17, 1981, assigned to the same assignee as the subject invention, and which is incorporated herein by reference.
As can be appreciated, the above described system, utilizing a piezoelectric crystal, is a "contact" technique requiring the attachment of the transducer to the sample. The latter requirement is time-consuming and potentially contaminating and is not suitable for production situations encountered in the semiconductor industry. Accordingly, there has been significant work carried out in developing noncontact detection techniques. One such noncontact detection technique is described in copending applications, Ser. No. 401,511, filed July 26, 1982, and now U.S. Pat. No. 4,521,118, issued June 4, 1985, and Ser. No. 481,275, filed Apr. 1, 1983 and now U.S. Pat. No. 4,522,510, issued June 11, 1985, both incorporated by reference.
The latter applications describe a method and apparatus for detecting thermal waves by monitoring the local angular changes occurring at the surface of the sample. More specifically, when thermal waves are generated in a localized region of the sample, the surface of the sample undergoes periodic angular changes within the periodically heated area because of local thermoelastic effects. These angular changes occur at a frequency equal to the frequency of the modulated heating beam. To monitor these changes, a beam of energy, such as a laser beam, is focused on the surface of the sample in a manner such that is reflected. Because of the local angular changes occurring at the surface of the sample, the reflected beam will experience angular displacements in a periodic fashion. By measuring the angular displacements, information about the thermal wave activity in the sample can be determined. The latter technique has proved to be a highly sensitive process for detecting thermal waves.
The subject invention, in contrast, is directed towards an independent and totally different method of detecting thermal waves. The technique disclosed herein may be used as an independent basis for the detection of thermal waves. In addition, when used in combination with any of the earlier described techniques, new and surprising additional information may be obtained about the characteristics of a sample. The advantages of using two different thermal wave detection techniques to gain additional information about a sample is described in detail in copending application, Ser. No. 612,077, filed May 21, 1984, assigned to the same assignee as the subject invention and incorporated herein by reference. These advantages are discussed briefly below.
In the above described techniques, such as monitoring the deflection of a probe beam or through detection of acoustic waves through a transducer, the output signals generated are primarily a function of the integral of the temperature distribution through the sample. In contrast, in the subject system, which is based on measurements of reflectivity, the output signals are primarily a function of surface temperature. The availability of two independent measurements of thermal wave signals permits the evaluation of both thickness and compositional variables in a sample. The latter concepts are set forth in detail, and are the subject of the copending application cited above, which is incorporated herein by reference. It should be understood, however, that the subject invention not only provides a new detection technique, but in addition, when combined with other measurement techniques, defines a completely new and powerful analytical tool with capabilities not found in the prior art.
Accordingly, it is an object of the subject invention to provide a new and improved apparatus and method for detecting thermal waves.
It is another object of the subject invention to provide a new and improved apparatus and method which detects thermal waves based on changes in reflectivity of the sample.
It is still a further object of the subject invention to provide a new and improved method and apparatus for detecting thermal waves which is based on surface temperature variations.